The present invention relates generally to improving the properties of a silicon carbide reinforced titanium aluminide matrix composite. More particularly, it relates to reducing the tendency of cracks to form in the titanium aluminide matrix.
It is known that filament strengthened composites can be formed by plasma deposition of a matrix material about a reinforcing filament. This teaching and related teachings are contained in the U.S. Pat. Nos. 4,775,547; 4,782,884; 4,786,566; 4,805,294; 4,805,833; and 4,838,337. The inventor of these prior art patents is one of the inventors herein and the prior art patents are assigned to the same assignee as the subject invention.
As pointed out in these earlier patents, it is known that silicon carbide fibers can be formed with great strength and with high temperature tolerance. It is also known that titanium foils have been used in connection with SiC fibers to produce SiC reinforced composites in which the SiC fibers are embedded in a sheet of titanium alloy made up of a number of layers of foil. The above-referenced patents are directed toward improvements over this conventional practice for forming silicon carbide reinforced matrices.
Employing the technique of the above-referenced patents, composites can be fabricated using several techniques pointed out in the patents to spray deposit any one of a variety of titanium base alloys on the silicon carbide reinforcing filaments. A preferred alloy for fabrication of such composites is a titanium base alloy containing 14 weight percent aluminum and 21 weight percent niobium. The alloy is known conventionally as Ti-1421. The matrix of the composite formed from such an alloy consists primarily of alpha-2, an ordered intermetallic phase with small amounts of beta-phase. The alpha-2 tends to have low ductility and envelopes of this phase around the SiC fiber have been found to crack during consolidation and also during subsequent thermal exposure. Radial cracks in the alpha-2 envelope propagate into the surrounding matrix when the material is loaded in tension. Such radial cracks may affect the overall mechanical properties by leading to premature composite fracture, and particularly lateral cracking and fracture.